Polyarylene sulfides are high-performance polymers that may withstand high thermal, chemical, and mechanical stresses and are beneficially utilized in a wide variety of applications. Polyarylene sulfides are generally formed via polymerization of a dihaloaromatic monomer with an alkali metal sulfide or an alkali metal hydrosulfide in an organic amide solvent.
In most formation processes, a mixture including the organic amide solvent, the alkali metal sulfide or alkali metal hydrosulfide, and water (and often an alkali metal hydroxide as well) are reacted to form a complex prior to the polymerization reaction with the dihaloaromatic monomer. Unfortunately, this reaction requires the presence of water, which is preferably removed prior to the polymerization reaction, and creates as by-product hydrogen sulfide that is formed according to the thermal decomposition equilibrium of the alkali metal sulfide:X2S+2H2O2H2S+XOHin which X is an alkali metal.
Loss of sulfur from the polymerization process can be quite high during this step, up to about 10% of the added sulfur in some systems. This can have detrimental effects on the product formed. For instance, when the loss of sulfur causes the ratios between other reactants and the sulfur (e.g., the diahaloaromatic monomer to sulfur ratio) to increase excessively, detrimental effects to the process and product can occur such as color changes, undesirable side reactions, malodor, as well as variations in molecular weight, melt viscosity, mechanical properties, and polymer end groups. Moreover, the gaseous hydrogen sulfide by-product can create toxicity in the process waste stream, which can have harmful effects on plant personnel as well as on the external environment and population if not properly treated according to additional, expensive waste processing.
Attempts have been made to address this problem. For instance, estimates of sulfur loss through the complex formation step have been made, with the input levels being adjusted accordingly. Unfortunately, this approach fails to prevent sulfur loss into the waste stream. In other attempts, the waste stream carrying sulfur in the form of hydrogen sulfide is brought into contact with the input reactor stream in an attempt to catch lost sulfur form the waste. In yet another attempt, the mixture being subjected to the complex formation process is subjected to elevated, declining pressure conditions to attain a more consistent and less extensive sulfur loss through the complex formation and dehydration step.
What is needed in the art is a method for prevention of sulfur loss during the formation of sulfur-containing polymers. For instance, methods for treatment of a reaction waste stream so as to capture sulfur from the waste and return the sulfur to the reaction process would be of great benefit.